My sourdough loaf is one big hole - help!

Hi!

I’m looking for some advice on the bake pictured – I want to try and bake again tomorrow and improve the disaster of today, but not sure which elements I’m getting wrong!

This is my fourth bake, and I loosely followed this video by Trevor Jay Wilson, Open Crumb from stiff sourdough. 65% hydration: https://youtu.be/WxfbiGto4R8

10g salt

290g water

24g wholewheat

436g Strong Canadian AP white

50g 100% hydration starter

My starter was built from my NMNF rye that I keep in the fridge, and woken and fed Weds PM until this morning Friday AM. I built it up to 100% hydration with AP white flour. It was bubbling away nicely and passed float test, had more than doubled in size within 3 hours (too young?) when I mixed. I did notice after an hour the little ball had sunk in the glass, but it did float for the whole time I was in the kitchen which was around 45 mins. 

My process was as follows and I’ll try and highlight where I felt things may have gone wrong, but all feedback welcome as all I want in life is to make my first good loaf and so far I’m not hitting the mark!

1. Per video, autolyse w/ salt overnight at room temp after few hours in fridge

2. Mix - ok so here I was a bit obsessed with gluten development. I’ve read so much about windowpane test that I got a bit obsessed with it and ended up mixing over about 2 hours - with breaks of 10 mins when dough tightened. I started using the method in the video but I couldn’t get the hang of it so switched to a few mins in my stand mixer with 10 min breaks x 3. When I left to BF properly it was loosely passing the pane test but did still split after a few seconds of glory. But I felt like I’d gone much overboard so stopped trying. I know this is way too long but the dough just didn’t feel pliable at all until I stopped?

3. BF at around 22C for another 3 hours. I couldn’t see any change in volume at all but dough sprung back slowly when poked and I was excited so

4. pre shaped and left for one hour - here I was planning to use round banneton so shaped into boule but

5. After final shape (which I was pretty pleased with as could see tension lines and felt very firm) I popped in floured banneton and realised the loaf didn’t even touch the sides so

6. Quickly popped out, reshaped into batard and into smaller batard banneton for 2.5 hour final prove again at around 22C

7. Dough not changing in volume still but still slow rise after poke so into the oven after a slash - I liberally covered in flour to aid scoring which I’m awful at but it still dragged a lot and I thing deflated somewhat?

(NB: I use a pre-heated cast iron casserole dish which means I usually end up dropping the loaf in from a height and I dropped it wonky today, and had to then drag it slightly to centre...!! Definitely didn’t help but I don’t have a proper Dutch oven)

8. Sprayed lid and baked covered for 20 mins and uncovered 20 mins at 225C (highest oven will go) until nicely brown and internal temp 100C

Please be as honest as possible, I can take it! I just want to learn where I’ve gone so wrong so I can hopefully have a better result tomorrow. The crust was great, thin and crispy, but no sour taste at all I think because no cold retard? Ideally I’d like a sourness but think I need to crack the basics first and this video seemed v detailed and low hydration as I’ve been baking at higher so far and can’t handle the dough properly.

thank you!!!3;